Electron ionization (EI) source is the most widely used ion source in traditional organic mass spectrometry. EI uses 70-eV electrons to impact organic molecules, and ionizes them to obtain characteristic spectrum for each organic compound, thus realizing accurate qualitative analysis. However, the mass peaks are seriously overlapped while analyzing complex organic mixtures, because of the high degree of fragmentation of organic compounds with EI. Therefore, it's difficult for EI to achieve rapid and online analysis. Vacuum ultraviolet (VUV) light can be used as a soft ion source for organic compounds with ionization energies (IEs) lower than the photon energy, and is especially propitious for rapid qualitative and quantitative analysis, due to its high molecular ion yield and very low degree of fragmentation. Keyong Hou et. al. (Chinese invention patent No. 200610011793.2) and Peichao Zheng et. al. (Chinese invention patent No. 200810022557.X) have reported the online mass spectrometry with VUV PI ion source. The achieved mass spectra in the patents are simple, including only the molecular ion peaks of the investigated organic compounds, and these features are beneficial to rapid qualitative and quantitative analysis.
The energies of photons emitted from VUV light sources are limited by the optical window materials used in the light sources. The highest photon energy that can be transmitted by the currently available optical window material is 11.8 eV, by using the optical window material of LiF. Thus, organic molecules with IEs lower than 11.8 eV can be effectively ionized by VUV PI. However, some organic and inorganic compounds, such as methane (CH4, IE=12.61 eV), acetonitrile (CH3CN, IE=12.20 eV), sulfur dioxide (SO2, IE=12.35 eV), and nitrous oxide (N2O, IE=12.89 eV), cannot be ionized by the VUV photons emitted from the existing VUV light sources. Therefore, the applications of VUV PI mass spectrometry subject to certain restrictions.